Use it as fuel. I have a few detractors in my life. Those that continually doubt your ability, tell you you 'can't' or 'aren't,' are those that most likely see their own shortcomings and wish to reflect those upon someone other than themselves. It is common for this type of bullying tactic to take place in the schoolyard, but when it is prevalent with those of a more mature age we have to question what might be holding them back. Here are two solutions to remedying this issue in the workplace:
1. Fuel for your Fire!
Place these fine words of 'encouragement' on your wall for motivation. Prove 'em wrong! Subtract the detractor from your equation by showing what you are made of. I do this all the time and it helps to build confidence over time.
2. Turn the Table!
No point in arguing - Ask the dark cloud in your life how they can help you succeed. Often times, they don't have anything to offer and this is a sure fire way to get them to realize they might not know everything as they think they do. If they are wise enough to recognize you are promoting a teamwork approach, maybe you can build a relationship of cooperation.
At the end of the day, it is you that make the difference in your output. If others try to keep you back, send them to the rear view mirror and keep looking forward,